When a large power step is suddenly applied to an alternating current (AC) motor, large torque transients may be developed. If the motor is connected in driving relationship to a load and the motor armature is rotating at some initial velocity, the sudden torque transient may result in damage or some other undesirable reaction on the load. Thus, it is desirable to apply power to a motor in a manner that avoids such undesirable effects. For a nonrotating motor, power may be simplied applied at some small voltage and low frequency to enable the motor to begin rotation. However, if the motor is spinning, i.e., the rotor is in motion, it is desirable to apply power at a voltage level and frequency which matches the motor speed in order to quickly gain control of motor operation. In particular, control systems utilizing volts per Hertz regulation have required knowledge of motor speed so as to properly adjust inverter frequency and converter voltage, to existing operating conditions. Should motor voltage be applied without first coordinating such voltage and frequency with motor conditions, excessive current and torque transients are known to result. Prior art solutions to this problem have required the use of a motor tachometer or other mechanical speed sensing devices to determine initial motor conditions.
Among the several objects of the present invention may be noted the provision of an improved method for starting a variable voltage inverter with a motor that may be pre-spinning; the provisions of this improved method including a means of achieving smooth starting conditions for an already spinning motor; the provision of such improved method in which both the volt-second set point and the DC link voltage are determined to match the existing motor rotational and excitation levels; the provision of such an improved method which renders mechanical monitoring devices unnecessary; the provision of an improved method which utilizes a voltage converter command in combination with an inverter volt-second command in order to gently achieve frequency lock consistent with the electrical frequency of a pre-spinning motor; the provisions of such improved method including the determination of voltage and frequency settings when a motor is spinning in the constant horsepower range; and the provisions of such method including reversal of inverter rotation phasing which is opposite to that required by a spinning motor.
In general an improved method is provided in one form of the invention for determining initial voltage and inverter frequency conditions in order to power a pre-spinning motor.